A tube rupture resulting in water flowing out in a combustor for combustion in a fluidized bed entails problems of a completely different dimension and difficulty than a tube rupture in an ordinary boiler, in which steam flowing out is evacuated with the flue gases to the chimney. A fluidized bed contains a large quantity of heat and a tube rupture involving water flowing out in the bed results in a violet steam generation. A gas turbine connected to the combustor limits the outflow of gas from the combustor so that a pressure increase may arise and lead to an impermissible pressure difference between the combustor and the surrounding space. From the point of view of safety, it is important to limit a pressure in the combustor, arising as a result of a tube rupture, by means of a pressure relief device. Because the combustor is enclosed in a pressure vessel and the pressure difference between the combustor and the pressure vessel varies with the load, special problems arise which must be taken into consideration when designing the relief system.